1. Field of the Invention
The invention relates to a magnetic motion sensor. More particularly, the present invention relates to a length or angle measuring system having a sensor array, comprising several individual sensor elements, and a scale, whose reluctance varies periodically along its length. The sensor may be used, for example, in automation or numerical control applications.
2. Description of Background Information
Examples of magnetic motion sensing devices are disclosed in German Patent No. DE-A3,214,794 and European Patent Document EP 0,093,232 (Heidenhain). In these example devices, the sensors are Hall elements which deliver as measuring signals a Hall voltage. An electronic system is provided for conditioning the measuring signals.
The electronic system is complicated and needs at least six compensation resistors. In addition, the thermal behavior of the compensation resistors is different than that of the Hall sensors which presents a problem.
Additionally, the width of the sensors in the measuring direction is limited to a maximum of t/4 where t is the scale pitch. Therefore, a good sinusoidal waveform cannot be obtained from the measuring signals. If the sensor width is smaller than t/4, the magnetic flux will not change due to a displacement amount of t/4 while the sensor remains adjacent the same bridge or perforation.
This means that such an instrument can detect the position of a sensor only for a unit. As an example, consider a field plate, which can be displaced along a tooth rack. The whole system represents a closed magnetic circuit. A displacement of the field plate relative to the tooth rack causes a resistance value to change periodically, and this varying resistance value is used to determine the position and speed of the movement of the field plate. To avoid or minimize measuring errors, a field concentrator, or pole piece, is used to increase the signal amplitude. This sensor unit is large in size and requires the provision of an appropriate piece for mounting the tooth rack on the machine.
In German Patent document DE-OS-3,926,328, a similar device is presented. For increased accuracy, the device places more magnetic pole shoes along the measuring direction. This device is intended for use in digital calipers. The device includes an oversized measuring head in order to obtain a usable output signal. (See, for example, FIGS. 1 and 6 of DE-OS-3,926,328).
Another similar device is disclosed in DE-OS-3,704,729. A pole piece is attached to the sensor, and, with its alternating flux, induces a magnetic field in a tooth rack, which serves at the scale. The device produces a displacement-dependent signal in accordance with the position of the measuring head relative to the tooth rack along the measuring direction. This device has a complicated construction, is too voluminous, and is very sensitive to the distance between the measuring head and the tooth rack. These motion sensing devices suffer from significant limitations. Some comprise a scale having a tooth rack, with limited accuracy, due to a large resolution and measuring length. Others comprise a scale made of a thin film divided structure on a dielectric substrate, which delivers poor measuring signals due to the limited inductive changes along the scale. The fabrication of all these device types is expensive.